The invention relates to a method and apparatus for reducing the moisture content bound by capillarity in fiber cells.
According to DE 195 35 315 A1, from which the invention sets out, a good, uniform permeability of the outspread bulk material in the process for the mechanical/thermal dewatering (MTE) of biological (fossil) and/or mineral bulk materials is the necessary requirement for a uniform thermal processing of the granulated bulk material spread out in the pressure chamber. For this purpose the coarsely granular raw material to be dewatered, with grain sizes of, for example,  greater than 0 to 50 mm, preferably  greater than 0 to 30 mm, is put through a process of comminution. Grain size is an optimal process parameter in regard to the ratio of grain surface to the grain volume in the transfer of the sensible heat of the hot process water from the previous press batch as well as the vapor condensation that follows in the course of the process. Depending on consistency, that is screen grain size distribution of the raw material to be ground with respect to the percentage of the fine grain to the coarse grain as well as the way in which the coarse lumps fracture, more or less fine material is produced with a grain size of less than one millimeter. A greater fine material content of  greater than 0 to 1 mm in the range of more than 6 to a maximum of 10% of the entire raw material as the bulk material, leads to uncontrollable unequal distribution of the fine material content in the poured bulk material. This unequal sifted grain size distribution leads to unequal resistance to the washing and therefore leads to pathways uncontrollably occurring in the bulk material within the pressure chamber. This unequal permeability interferes with the uniform transfer of heat to the granular material. This irregular thermal processing of the bulk material causes very irregular and unacceptable degrees of moisture reduction, so that the process as such and the economy of the process are cast into doubt.
Thus it has been found in the practice of such processes and the use of apparatus for raw brown coal with a capillarity-bound moisture content of about 55 to 65 weight-percent, that
with grain sizes  less than 1 mm and a percentage in the bulk of more than 6 to 10% of the bulk, due to uneven permeability, pockets of moisture are still present after the dewatering process,
with grain sizes  less than 1 mm and a content in the bulk of less than 10%, even with an irregular sifted grain distribution or separation effects in the bulk, uniform permeability and thus a uniform and good dewatering is achieved, and
with a reduction of the content of the grain sizes under 1 mm to xe2x89xa66%, the degree of dewatering is markedly improved, up to xe2x89xa620%.
In the screening of the material to be dewatered by the former methods it was a disadvantage that the fine materials above the critical content or critical permeability limit (i.e. 10% of the bulk content having grain sizes of less than 1 mm) with up to 35% residual fines content of the entire material could not be fed back to the MTE process.
For the solution of the problem described, the purpose of the invention is to create an optimum screened grain size distribution for the bulk material and establish it so that a heretofore unacceptable high fines content can be fed to the MTE material without interfering with a good, uniform dewatering process.
The solution of this problem consists in that the input material is processed by screening and comminution and fed separately to several spreader hoppers, wherein, in a first part, the coarse material is separately fed to a first spreader hopper such that the coarse material contains only a measured residual amount of fine material below the critical permeability limit, while at the same time the fines thus separated with a grain size of  less than 3 to  greater than 0 mm are fed to a second spreader hopper, while the second spreader hopper which is ahead of the first coarse material spreader hopper spreads a first thin layer of fines on the spreading, feeding and filter belt and then a substantially thicker coarse material layer from the first spreader hopper is laid on as the second layer to form a sandwich of spread material, while the application of the fines depth HF and coarse material depth HG of the sandwich is performed corresponding to the consistency and the percentage content of the fines of the input material used, and the sandwiched layer formed in the two preceding steps is delivered by the spreading, feeding and filter belt into the MTE pressure chamber of the filter press according to the dewatering cycle, while at the same time the squeezed-out dry material is discharged.
In a first embodiment of an apparatus for the practice of the method, the problem is solved in that the arrangement of several hoppers in tandem, wherein the bulk material is treated by comminution and screening, the coarse material is transferred into a first spreading hopper, and the screened-out fines are carried over a chute and conveyor belt into a second spreading hopper. The material is then passed through a system in which the fines can be deposited first in a thin layer and the coarse material is deposited from the spreading hoppers in a thick layer thereon through sliding gates controlling the depth of the layer. The deposited material is then passed onto the spreading, feeding and filter belt so as to form a sandwich.
A second embodiment of an apparatus according to claim 8 divides the material by arranging a plurality of screening devices connected in tandem into ever decreasing grain sizes and it feeds the material on conveyor belts into spreading hoppers arranged separately in tandem, so that the fines and the superfines can be applied to the spreading, feeding and filter belt with the smallest grain size first in two thin layers, and thereafter the coarse material can be applied thereon to form a sandwich of three layers.
By the method of the invention it is accomplished that the input material, prepared by screening and comminution, is divided into fractions such that the material will be in a sandwich of the material with a grain size distribution that assures optimum permeability and improved heat treatment of each layer of the material.
It is advantageous that the hopper or hoppers for the fines are arranged ahead of the hopper for the coarse material, so that the thin layer spread with the fines will come to lie as a first layer on the spreading and feeding belt, and the substantially thicker coarse material layer will lie on top of it. The depth of the layers is to be controlled according to the amount of the fines that will be best for the process. Due to the structure of the sandwich of material, the hot process water front E will flow uniformly first through depth HG of the coarse material layer, which will be about 65% to 90% of the total depth xcexa3H. The hot process water at about 200 to 220xc2x0 C. is driven through by the steam pressure of about 16 bar to 24 bar acting on the hot water column E. Both of the media, hot water E and steam D, are fed downward into the material in the pressure chamber through the nozzles of the spreading system. As the hot water front E passes through, its perceptible heat is yielded to the cold material which is at about 20xc2x0 C. room temperature. Along the steam front D the saturated steam condenses on the grain surfaces. The layers of material thus have a permeability that is optimum for each. Due to the sandwich structure of coarse and fine layers there can be no concentration of fines in the depth HG of the coarse material. The screened fine material is controlled at a relatively low depth HF=10-35% of the total depth xcexa3H above the bottom dewatering filter surface of the MTE pressure chamber. Due to the controlled spreading of the fines a uniform permeability is present in the fines, which assures an even flow across their area of the media fronts D and E and prevents any uncontrolled separation in the coarse and fine regions due to the separate layers in the layer depths HG and HF1+HF2.
Due to the measured sifting out and processing of the fines content from the coarse material content of the starting material and the subsequent separate flow through the layers HG, HF, and HF1+HF2 there can be no intermixing of the coarse and fine granules while they are being spread on the spreading and feeding belt. Thus the dewatering efficiency in raw brown coal with a 55 to 65 weight-percent moisture content can be improved down to about 18% residual moisture content, including post-evaporation for dry brown coal. That is 25% more dewatering performance than practiced heretofore in the MTE process, which leads to a further increase of the heat value of the dry brown coal.
The higher dewatering power is due to the improved utilization of the perceptible heat in the recirculation of the MTE process water of 200 to 220xc2x0 C., because the greater resistance offered by the fines layers to its passage produces a more intensive heat transfer to the surfaces of the grains. On account of this, the MTE process water is advantageously substantially cooler at about 30xc2x0 C. at the filter belt surface upon exiting after its thermal energy is absorbed. The layers of the the fines furthermore have advantageously the action of a xe2x80x9ccarbon filter,xe2x80x9d so that the cold process water at about 30xc2x0 C. is carried away as virtually clear water at the filter belt surface, that is, solid matter is filtered out more effectively and the cost of purifying the process water is thus markedly reduced. In comparison, in the formerly known MTE process, brown-colored water is carried out on the filter belt surface with an elevated solids content.
Additional advantageous measures and embodiments of the subject matter of the invention will be found in the subordinate claims and in the following description and drawing.